Cryo-EM structure of 90 S small ribosomal subunit precursors in transition states

Science. 2020 Sep 18;369(6510):1477-1481. doi: 10.1126/science.aba9690.


The 90S preribosome is a large, early assembly intermediate of small ribosomal subunits that undergoes structural changes to give a pre-40S ribosome. Here, we gained insight into this transition by determining cryo-electron microscopy structures of Saccharomyces cerevisiae intermediates in the path from the 90S to the pre-40S The full transition is blocked by deletion of RNA helicase Dhr1. A series of structural snapshots revealed that the excised 5' external transcribed spacer (5' ETS) is degraded within 90S, driving stepwise disassembly of assembly factors and ribosome maturation. The nuclear exosome, an RNA degradation machine, docks on the 90S through helicase Mtr4 and is primed to digest the 3' end of the 5' ETS. The structures resolved between 3.2- and 8.6-angstrom resolution reveal key intermediates and the critical role of 5' ETS degradation in 90S progression.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cryoelectron Microscopy
  • DEAD-box RNA Helicases / chemistry*
  • DEAD-box RNA Helicases / genetics
  • DEAD-box RNA Helicases / metabolism
  • Exosomes / metabolism
  • Gene Deletion
  • Protein Domains
  • RNA Stability*
  • RNA, Ribosomal, 18S / chemistry
  • RNA, Ribosomal, 18S / metabolism
  • Ribosome Subunits, Large, Eukaryotic / chemistry*
  • Ribosome Subunits, Large, Eukaryotic / metabolism
  • Ribosome Subunits, Small, Eukaryotic / chemistry*
  • Ribosome Subunits, Small, Eukaryotic / metabolism
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins / chemistry*
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism


  • RNA, Ribosomal, 18S
  • Saccharomyces cerevisiae Proteins
  • ECM16 protein, S cerevisiae
  • MTR4 protein, S cerevisiae
  • DEAD-box RNA Helicases